clinical studies have defined a form of limited immunosuppression, in patients with chronic schistosomiasis (with S. mansoni or S. mekongi that develops progressively after initial infection. This immune suppression is characterized by poor or absent lymphocyte to proliferative responsiveness to parasite antigens in vitro and diminished immediate hypersensitivity (IH) responses to parasite antigen in vivo. The lymphocyte suppression is effected by various serum factors, by adherent mononuclear cells and by histamine released from the patients' IgE-coated basophils. IH responses are modulated by IgG 'blocking antibodies' directed against the same immunogens that evade IgE antibody responses. Long-term follow-up of treated patients indicates that these suppressive mechanisms diminish after therapy.